Versatile, facile and scalable route to polylactic acid-backbone graft and bottlebrush copolymers

ABSTRACT

Polylactic acid-backbone graft and bottlebrush copolymers are synthesized by polymerizing a lactide-functionalized macromonomer using ring opening polymerization (ROP). In some embodiments of the present invention, the macromonomer is a lactide-functionalized polymer that may be synthesized by, for example, polymerizing a monomer capable of undergoing radical polymerization (e.g., styrenic, vinylic, acrylic, etc.) using a brominated lactide initiator via atom transfer radical polymerization (ATRP). The brominated lactide initiator may be 3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione prepared by, for example, reacting lactide with N-bromosuccinimide in the presence of benzoyl peroxide.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is a continuation application of pending U.S.patent application Ser. No. 14/243,191 (docket no. ROC920130289US1),filed Apr. 2, 2014, entitled “VERSATILE, FACILE AND SCALABLE ROUTE TOPOLYLACTIC ACID-BACKBONE GRAFT AND BOTTLEBRUSH COPOLYMERS”, which ishereby incorporated herein by reference in its entirety.

BACKGROUND

The present invention relates in general to the field of biobasedmaterials. More particularly, the present invention relates topolylactic acid-backbone graft and bottlebrush copolymers prepared fromlactide-functionalized macromonomers using ring opening polymerization(ROP).

SUMMARY

In accordance with some embodiments of the present invention, polylacticacid-backbone graft and bottlebrush copolymers are synthesized bypolymerizing a lactide-functionalized macromonomer using ring openingpolymerization (ROP). In some embodiments of the present invention, themacromonomer is a lactide-functionalized polymer that may be synthesizedby, for example, polymerizing a monomer capable of undergoing radicalpolymerization (e.g., styrenic, vinylic, acrylic, etc.) using abrominated lactide initiator via atom transfer radical polymerization(ATRP). The brominated lactide initiator may be3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione prepared by, for example,reacting lactide with N-bromosuccinimide in the presence of benzoylperoxide.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Embodiments of the present invention will hereinafter be described inconjunction with the appended drawings, where like designations denotelike elements.

FIG. 1 is a graphical depiction of a bottlebrush copolymer having apolylactic acid-backbone and polymer (e.g., styrenic, vinylic, acrylic,etc.) grafts.

FIG. 2 is graphical depiction of a graft copolymer having a polylacticacid-backbone and polymer (e.g., styrenic, vinylic, acrylic, etc.)grafts.

FIG. 3 is a chemical reaction diagram showing the preparation of apolylactic acid-backbone bottlebrush copolymer from thelactide-functionalized polymer3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione using ring openingpolymerization in accordance with some embodiments of the presentinvention.

FIG. 4 is a chemical reaction diagram showing the preparation of apolylactic acid-backbone graft copolymer from lactide-functionalizedpolymer 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione andnon-functionalized lactide using ring opening polymerization (ROP) inaccordance with some embodiments of the present invention.

DETAILED DESCRIPTION

The depletion of fossil fuels from which the majority of polymers arederived, combined with supply chain instability and cost fluctuations offeed chemicals used to make these polymers, is driving the developmentand utilization of biobased plastics for commodity applications.Polylactic acid (PLA), derived from starch and sugars, is a particularlyappealing biobased plastic that is inexpensive and already beingproduced in large commercial quantities. In comparing polymers' materialproperties, polystyrene is often considered the petrochemical-basedcounterpart to PLA. Thus PLA is capable of replacing manypetroleum-derived polymers in some applications. However, several ofPLA's material properties—such as low heat-distortion temperature, highwater adsorption, low flame retardancy, and low toughness—exclude theuse of PLA in many applications. Moreover, additives to improve suchproperties are often expensive and/or come at the cost of sacrificingPLA's beneficial material properties. Hence much effort has beendedicated to directly incorporate chemical functionalities into thebackbone of PLA (or PLA's monomer lactide) in order to tailor itsproperties, but because of the chemical lability of both lactide andPLA, examples of such chemical modification in the prior art typicallyare complex and low-yielding. A versatile and high-yielding approach tomodify either PLA or lactide would greatly expand the profitability ofPLA by virtue of its extension to many new end uses.

For purposes of this document, including the claims, the term “lactide”includes all stereoisomers of lactide (e.g., (S,S)-lactide,(R,R)-lactide, and (S,R)-lactide). (S,S)-lactide is also referred to as“L-lactide”. (R,R)-lactide is also referred to as “D-lactide”.(S,R)-lactide is also referred to as “meso-lactide”. A racemic mixtureof D-lactide and L-lactide is often referred to as “DL-lactide”.

In accordance with some embodiments of the present invention,lactide-functionalized macromonomers are used to form bottlebrush andgraft copolymers with PLA backbones. Lactide-functionalizedmacromonomers utilized in this regard may be lactide-functionalizedpolymers with a lactide endgroup and a polymer backbone chosen to tailormaterial properties of the overall copolymer. That is, lactide (PLA'smonomer) can be functionalized with a wide array of different polymersdesigned to engineer specific properties to bottlebrush and graftcopolymers. This extends the use of PLA to applications not previouslypossible and creates new markets for PLA.

Lactide-functionalized macromonomers can be polymerized either alone toform PLA-backbone bottlebrush copolymers (see FIG. 1, described below)or in the presence of non-functionalized lactide to form PLA-backbonegraft copolymers (see FIG. 2, described below). PLA-backbone bottlebrushcopolymers synthesized in accordance with some embodiments of thepresent invention have a relatively high density of grafted polymers,while PLA-backbone graft copolymers synthesized in accordance with someembodiments of the present invention have a relatively low density ofgrafted polymers. PLA-backbone bottlebrush copolymers and PLA-backbonegraft copolymers synthesized in accordance with some embodiments of thepresent invention are well defined and controllable with lowpolydispersities (e.g., PDI<1.5).

PLA-backbone bottlebrush copolymers and PLA-backbone graft copolymerssynthesized in accordance with some embodiments of the present inventionconstitute chemically-functionalized PLA, and the polymers (e.g.,styrenic, vinylic, acrylic, etc.) bonded to the PLA can be strategicallychosen to engineer various desired properties in to the overallcopolymer. Furthermore, covalent bonding of other polymers to PLA, as inthe PLA-backbone bottlebrush and graft copolymers synthesized inaccordance with some embodiments of the present invention, has theadditional advantage of forming micro- and nano-structured polymers,resulting from phase separation of the two chemically bonded polymericcomponents. Micro- and nano-scale phase separation of immisciblepolymers results in maximized load transfer between the two phases,thereby optimizing the positive effect of the macromonomer on theoverall copolymer.

Micro- and nano-structured polymers are formed, in accordance with someembodiments of the present invention, by a simple annealing process(i.e., heating) of the PLA-backbone bottlebrush and graft copolymers.This simple annealing process results in phase separation of the twopolymeric components of the PLA-backbone bottlebrush and graftcopolymers.

FIG. 1 is a graphical depiction of a bottlebrush copolymer having apolylactic acid-backbone and polymer (e.g., styrenic, vinylic, acrylic,etc.) grafts. In FIG. 1, the PLA-backbone is depicted with a solid lineand the grafted polymers are depicted with dashed lines. As noted above,PLA-backbone bottlebrush copolymers synthesized in accordance with someembodiments of the present invention have a relatively high density ofgrafted polymers.

FIG. 2 is graphical depiction of a graft copolymer having a polylacticacid-backbone and polymer (e.g., styrenic, vinylic, acrylic, etc.)grafts. In FIG. 2, the PLA-backbone is depicted with a solid line andthe grafted polymers are depicted with dashed lines. As noted above,PLA-backbone graft copolymers synthesized in accordance with someembodiments of the present invention have a relatively low density ofgrafted polymers.

The polydispersity index (PDI) is a measure of the distribution ofmolecular mass in a given polymer sample. PDI is defined as M_(w)/M_(n),where M_(w) is the weight-average molecular weight and M_(n) is thenumber-average molecular weight. PLA-backbone bottlebrush copolymers andPLA-backbone graft copolymers synthesized in accordance with someembodiments of the present invention have low PDI (e.g., PDI<1.5).

A simple, two-step method may be employed to chemically modify lactide(PLA's monomer) in such a way that it can be functionalized with a widearray of different polymers designed to engineer specific properties toPLA. Brominated lactide (which may be formed in a one-step process fromlactide monomer) can be used directly to initiate polymerization of avariety of monomers through a well-known, often-utilized process calledatom-transfer radical polymerization (ATRP). This results in alactide-functionalized polymer, i.e., a lactide molecule that isfunctionalized with a polymer. By using lactide as an ATRP-basedinitiator, it is possible to form well-defined, “living”, and lowpolydispersity index (PDI) polymers. Hence, only two well-defined,high-yielding chemical reactions are required to synthesize alactide-functionalized polymer.

ATRP is a polymerization technique that is well known to those skilledin the art. ATRP is a controlled “living” free radical polymerizationtechnique. A low concentration of active radicals is maintained topromote slow growth of the molecular weight and, hence, the “living”ATRP process is controlled. Lactide-functionalized polymers synthesizedvia ATRP are “living” polymers in the same sense. These polymers presentno inherent termination mechanism.

In accordance with some embodiments of the present invention,lactide-functionalized polymers are used as macromonomers. Generally,macromonomers are oligomers with a number-average molecular weight M_(n)between about 1000 and about 10,000 that contain at least one functionalgroup suitable for further polymerization.

A lactide-functionalized polymer may be synthesized by ATRP of a monomercapable of undergoing radical polymerization (e.g., styrenic, vinylic,acrylic, etc.) using 3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione as abrominated lactide initiator. 3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dionemay be prepared by, for example, reacting lactide withN-bromosuccinimide (NBS) in the presence of benzoyl peroxide. Oneskilled in the art will appreciate, however, that3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione may be prepared using anynumber of methods known to those skilled in the art. For example,3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione may be prepared by reactinglactide with bromine (Br₂) in the presence of benzoyl peroxide.

Lactide is a commercially available biobased cyclic ester monomer thatcan be obtained from biomass. Lactide is the cyclic di-ester of lacticacid. Lactide may be prepared by heating lactic acid in the presence ofan acid catalyst. Lactide is a solid at room temperature. The meltingpoint temperature of each of L-lactide and D-lactide is between 95 and97° C. Racemic lactide has a melting point temperature between 116 and119° C. The melting point temperature of meso-lactide is less than 60°C. (˜53° C.).

The brominated lactide initiator, in the presence of a copper (I) and,optionally, copper (II) complex, an appropriate ligand (e.g.,N,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA)) and a monomercapable of undergoing radical polymerization (e.g., styrenic, vinylic,acrylic, etc.) undergoes an ATRP reaction to form alactide-functionalized polymer with a polymer backbone (the identity ofpolymer may be chosen to tailor material properties) and a lactideendgroup capable of, for example, being polymerized through traditionalPLA synthetic methods or using as a standalone initiator. As anillustrative example, polymerization of styrene via ATRP may beperformed in tetrahydrofuran (THF) at 60-70° C. In this example, theconcentration of styrene may be approximately 1.6 M and the ratio ofstyrene to 3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione may beapproximately 200. Alternatively, the ATRP reaction may be performed ina melt state (e.g., no solvent) using melt polymerization. Meltpolymerization techniques are well known in the art.

As noted above, ATRP is a polymerization technique that is well known tothose skilled in the art. ATRP can be used with myriad differentmonomers to produce myriad different polymers without undueexperimentation. Generally, polymerization via ATRP is conducted underextremely low steady state concentration of active radicals, allowingpropagation of the polymer chain to proceed with suppressedradical-radical coupling. For example, the monomer and initiator may beadded to a solution containing a catalytic copper/ligand complex (i.e.,an ATRP catalyst and a ligand). Exemplary ATRP catalysts include, butare not limited to, copper(I) complexes such as copper(I) bromide (CuBr)and, optionally, copper(II) complexes such as copper(II) dibromide(CuBr₂). Traditional ATRP can be done with added copper (II), but stillmust have some copper (I) added. Exemplary ligands include, but are notlimited to, bipyridines such as 4,4′-dinonyl-2,2′bipyridine (DNBP) andbi-, tri- and tetradentate amines such asN,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA).

The catalytic copper/ligand complex may be deoxygenated using knowntechniques such as successive cycles of freeze-pump-thaw. One skilled inthe art will appreciate, however, that other techniques fordeoxygenating the mixture may be used in lieu of, or in addition to,successive cycles of freeze-pump-thaw.

The ratio of ATRP catalyst (e.g., CuBr) to monomer (e.g., styrene) canvary, although suitable results are obtained with ratios of 10:1-50:1.The ratio of monomer (e.g., styrene) to initiator (e.g.,3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione) may also vary, althoughratios of about 1:10-1:200 (or more) provide suitable results.

The ATRP synthesis of the lactide-functionalized polymer is performed atan appropriate temperature, for example, 60-70° C. The appropriatetemperature can vary, however, depending on a number of factorsincluding, but not limited to, the identity of the monomer, theinitiator, the ATRP catalyst, and the ligand, as well as the boilingpoint of the solvent, if any.

The order of addition of the reagents can have a profound affect on theinitiator efficiency. To optimize this, the copper/ligand complex mustbe formed with a slight excess of copper prior to exposure to thebrominated lactide initiator.

PLA-backbone bottlebrush copolymers and PLA-backbone graft copolymersare synthesized, in accordance with some embodiments of the presentinvention, by using the lactide-functionalized polymer as a macromonomerin a well-known, often-utilized process called ring openingpolymerization (ROP). Lactide-functionalized macromonomer is polymerizedeither alone (Reaction Scheme 1, described below) to form PLA-backbonebottlebrush copolymers or in the presence of lactide (Reaction Scheme 2,described below) to form PLA-backbone graft copolymers. Variouscatalysts well known in PLA polymerization can be utilized in thepolymerization of the lactide-functionalized macromonomer. Exemplarycatalysts include, but are not limited to, tin(II) 2-ethylhexanoate(Sn(Oct)₂) (also referred to as “stannous octoate” and “tin octoate”),dimethylaminopyridine (DAP), diazabicycloundecene (DBU), and the like.

As noted above, ROP is a polymerization technique that is well known tothose skilled in the art. Generally, both metal and metal-free catalystsmay be used in ROP polymerizations. The catalyst facilitates acoordination-insertion with the carbonyl portion of thelactide-functionalized macromonomer (and, optionally, lactide ifsynthesizing a PLA-backbone graft copolymer) and the hydroxyl group ofan available alcohol. The ring-opening of the lactide-functionalizedmacromonomer (and, optionally, lactide if synthesizing a PLA-backbonegraft copolymer) by the available alcohol results in the availability ofanother alcohol for further polymerization.

Reaction Scheme 1, described below, is a general synthetic example ofthe polymerization to synthesize PLA-backbone bottlebrush copolymers inaccordance with some embodiments of the present invention. In the firststep of Reaction Scheme 1, brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione is prepared by reactinglactide with N-bromosuccinimide (NBS) in the presence of benzoylperoxide. In the second step of Reaction Scheme 1, alactide-functionalized polymer is obtained by ATRP of a monomer capableof undergoing radical polymerization (e.g., styrenic, vinylic, acrylic,etc.) initiated from the brominated lactide monomer in the presence of acopper (I) complex/PMDETA. In the third step of Reaction Scheme 1, aPLA-backbone bottlebrush copolymer is obtained by ROP using thelactide-functionalized polymer as a macromonomer.

In the second and third steps of Reaction Scheme 1, R is a hydrogen atomor a methyl group, and R′ is a phenyl group or C(O)OR″, wherein R″ is analkyl group having one or more carbon atoms. Suitable examples ofmonomers capable of undergoing radical polymerization in the second stepof Reaction Scheme 1 include, but are not limited to, styrene, butylacrylate, methyl acrylate, 2-ethylhexyl acrylate, ethyl acrylate,2-ethylhexl methacrylate, ethyl methacrylate, butyl methacrylate, andcombinations thereof.

Lactide-functionalized polymers (used as macromonomers in the third stepof Reaction Scheme 1) may be synthesized using L-lactide as the startingmaterial. In the first step of Reaction Scheme 1, a bromine addition onthe L-lactide is employed to synthesize brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione. In the second step ofReaction Scheme 1, a monomer capable of undergoing radicalpolymerization is polymerized via ATRP using the brominated lactidemonomer as an initiator.

In the first step of Reaction Scheme 1, a mixture of L-lactide, benzeneand N-bromosuccimide (NBS) are added to a three-neck flask and heated toreflux. Generally, stoichiometric amounts of L-lactide and NBS are used.Mechanical stirring is employed throughout reflux. A solution of benzoylperoxide in benzene is then added dropwise over time through a droppingfunnel, syringe or other suitable technique. Generally, any catalyticamount of benzoyl peroxide may be used. One skilled in the art willappreciate that any suitable solvent may be used in these solutions inlieu, or in addition to, benzene. Suitable solvents include, but are notlimited to, benzene and acetonitrile. After the monomer is consumed, thereaction mixture is cooled to room temperature. The reaction product,which is brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione, may be purified usingtechniques well known in the art.

In the second step of Reaction Scheme 1, CuBr andN,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA) are added to a firstflask, along with a magnetic stirrer. The first flask is fitted with arubber septum and degassed with three successive cycles offreeze-pump-thaw. Generally, the catalytic complex must be formed with aslight excess of copper ([Cu]₀/[PMDETA]₀>1) before exposure to thelactide initiator. Providing a slight excess of copper preventsundesirable side reactions. To a second flask are added some of the3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione prepared in the first step ofReaction Scheme 1, THF, and a monomer capable of undergoing radicalpolymerization. Generally, the ratio of[monomer]₀/[3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione]₀ that may beused ranges from 1:10-1:200 (or more). The second flask is fitted with arubber septum and degassed by bubbling with N₂ flow for at least 30minutes. This mixture is then transferred into the first flask and thepolymerization is carried out under stirring at a suitable temperature.Polymerization typically occurs over a period of hours. Generally, thepolymerization of the monomer via ATRP may be performed in THF at 60-70°C. The reaction product, which is lactide-functionalized polymer3-poly(monomer)-3,6-dimethyl-1,4-dioxane-2,5-dione, may be purifiedusing techniques well known in the art.

One skilled in the art will appreciate that any suitable catalyticcomplex may be used in lieu, or in addition to, CuBr/PMDETA catalyticcomplex. Suitable catalytic complexes include both a suitable ATRPcatalyst and a suitable ligand. Suitable ATRP catalysts include, but arenot limited to, copper(I) complexes such as CuBr or other copperhalides. Suitable ligands include, but are not limited to, bipyridinesand bi-, tri- and tetradentate amines. Specific examples of suitableligands include 4,4′-dinonyl-2,2′bipyridine (DNBP),N,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA), and1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA).

In the third step of Reaction Scheme 1, a solution of stannous octoate(Sn(Oct)₂) in anhydrous toluene and a solution of benzyl alcohol inanhydrous toluene are added to a flask, and the solvent is removed invacuo. Generally, any catalytic amount of Sn(Oct)₂ or other suitablecatalyst may be used. A similar amount of benzyl alcohol or othersuitable initiator is typically used. One skilled in the art willappreciate that any suitable solvent may be used in the Sn(Oct)2solution and benzyl alcohol solution in lieu, or in addition to,anhydrous toluene. Some of the lactide-functionalized polymer(macromonomer) prepared in the second step of Reaction Scheme 1 is addedto the flask, along with a magnetic stirrer. The flask is fitted with arubber septum and degassed by bubbling with N₂ flow for at least 30minutes. The polymerization is carried out under stirring at a suitabletemperature. Polymerization typically occurs over a period of hours.Generally, the polymerization of the lactide-functionalized macromonomervia ROP may be performed in toluene at 110° C. Alternatively, the ROPreaction may be performed in a melt state (e.g., no solvent) at 110-180°C. using melt polymerization. Melt polymerization techniques are wellknown in the art. The reaction product, which is a PLA-backbonebottlebrush copolymer, may be purified using techniques well known inthe art.

One skilled in the art will appreciate that any suitable catalyst may beused in lieu, or in addition to, Sn(Oct)₂. Generally, both metal andmetal-free catalysts may be used. Suitable catalysts include, but arenot limited to, Sn(Oct)₂, dimethylaminopyridine (DAP),1,8-diazabicycloundec-7-ene (DBU), and1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).

One skilled in the art will appreciate that any initiator may be used inlieu, or in addition to, benzyl alcohol. Suitable initiators include,but are not limited to, benzyl alcohol, primary alcohols (e.g., ethanoland butanol), 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate.

Reaction Scheme 2, described below, is a general synthetic example ofthe polymerization to synthesize PLA-backbone graft copolymers inaccordance with some embodiments of the present invention. In the firststep of Reaction Scheme 2, brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione is prepared by reactinglactide with N-bromosuccinimide (NBS) in the presence of benzoylperoxide. In the second step of Reaction Scheme 2, alactide-functionalized polymer is obtained by ATRP of a monomer capableof undergoing radical polymerization (e.g., styrenic, vinylic, acrylic,etc.) initiated from the brominated lactide monomer in the presence of acopper (I) complex/PMDETA. In the third step of Reaction Scheme 2, aPLA-backbone graft copolymer is obtained by ROP using and thelactide-functionalized polymer as a macromonomer and non-functionalizedlactide.

In the second and third steps of Reaction Scheme 2, R is a hydrogen atomor a methyl group, and R′ is a phenyl group or C(O)OR″, wherein R″ is analkyl group having one or more carbon atoms. Suitable examples ofmonomers capable of undergoing radical polymerization in the second stepof Reaction Scheme 2 include, but are not limited to, styrene, butylacrylate, methyl acrylate, 2-ethylhexyl acrylate, ethyl acrylate,2-ethylhexl methacrylate, ethyl methacrylate, butyl methacrylate, andcombinations thereof.

Lactide-functionalized polymers (used as macromonomers in the third stepof Reaction Scheme 2) may be synthesized using L-lactide as the startingmaterial. In the first step of Reaction Scheme 2, a bromine addition onthe L-lactide is employed to synthesize brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione. In the second step ofReaction Scheme 2, a monomer capable of undergoing radicalpolymerization is polymerized via ATRP using the brominated lactidemonomer as an initiator.

In the first step of Reaction Scheme 2, a mixture of L-lactide, benzeneand N-bromosuccimide (NBS) are added to a three-neck flask and heated toreflux. Generally, stoichiometric amounts of L-lactide and NBS are used.Mechanical stirring is employed throughout reflux. A solution of benzoylperoxide in benzene is then added dropwise over time through a droppingfunnel, syringe or other suitable technique. Generally, any catalyticamount of benzoyl peroxide may be used. One skilled in the art willappreciate that any suitable solvent may be used in these solutions inlieu, or in addition to, benzene. Suitable solvents include, but are notlimited to, benzene and acetonitrile. After the monomer is consumed, thereaction mixture is cooled to room temperature. The reaction product,which is brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione, may be purified usingtechniques well known in the art.

In the second step of Reaction Scheme 2, CuBr andN,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA) are added to a firstflask, along with a magnetic stirrer. The first flask is fitted with arubber septum and degassed with three successive cycles offreeze-pump-thaw. Generally, the catalytic complex must be formed with aslight excess of copper ([Cu]₀/[PMDETA]₀>1) before exposure to thelactide initiator. Providing a slight excess of copper preventsundesirable side reactions. To a second flask are added some of the3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione prepared in the first step ofReaction Scheme 2, THF, and a monomer capable of undergoing radicalpolymerization. Generally, the ratio of[monomer]₀/[3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione]₀ that may beused ranges from 1:10-1:200 (or more). The second flask is fitted with arubber septum and degassed by bubbling with N₂ flow for a few minutes.This mixture is then transferred into the first flask and thepolymerization is carried out under stirring at a suitable temperature.Polymerization typically occurs over a period of hours. Generally, thepolymerization of the monomer via ATRP may be performed in THF at 60-70°C. The reaction product, which is lactide-functionalized polymer3-poly(monomer)-3,6-dimethyl-1,4-dioxane-2,5-dione, may be purifiedusing techniques well known in the art.

One skilled in the art will appreciate that any suitable catalyticcomplex may be used in lieu, or in addition to, CuBr/PMDETA catalyticcomplex. Suitable catalytic complexes include both a suitable ATRPcatalyst and a suitable ligand. Suitable ATRP catalysts include, but arenot limited to, copper(I) complexes such as CuBr or other copperhalides. Suitable ligands include, but are not limited to, bipyridinesand bi-, tri- and tetradentate amines. Specific examples of suitableligands include 4,4′-dinonyl-2,2′bipyridine (DNBP),N,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA), and1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA).

In the third step of Reaction Scheme 2, a solution of stannous octoate(Sn(Oct)₂) in anhydrous toluene and a solution of benzyl alcohol inanhydrous toluene are added to a flask, and the solvent is removed invacuo. Generally, any catalytic amount of Sn(Oct)₂ or other suitablecatalyst may be used. A similar amount of benzyl alcohol or othersuitable initiator is typically used. One skilled in the art willappreciate that any suitable solvent may be used in the Sn(Oct)2solution and benzyl alcohol solution in lieu, or in addition to,anhydrous toluene. Some of the lactide-functionalized polymer(macromonomer) prepared in the second step of Reaction Scheme 2 andnon-functionalized lactide are added to the flask, along with a magneticstirrer. Generally, the amount of lactide-functionalized macromonomerrelative to the amount of non-functionalized lactide may be adjusted toachieve a desired density of grafted polymers. The flask is fitted witha rubber septum and degassed by bubbling with N₂ flow for at least 30minutes. The polymerization is carried out under stirring at a suitabletemperature. Polymerization typically occurs over a period of hours.Generally, the polymerization of the lactide-functionalized macromonomerand non-functionalized lactide via ROP may be performed in toluene at110° C. Alternatively, the ROP reaction may be performed in a melt state(e.g., no solvent) at 110-180° C. using melt polymerization. Meltpolymerization techniques are well known in the art. The reactionproduct, which is a PLA-backbone graft copolymer, may be purified usingtechniques well known in the art.

One skilled in the art will appreciate that any suitable catalyst may beused in lieu, or in addition to, Sn(Oct)₂. Generally, both metal andmetal-free catalysts may be used. Suitable catalysts include, but arenot limited to, Sn(Oct)₂, dimethylaminopyridine (DAP),1,8-diazabicycloundec-7-ene (DBU), and1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD).

One skilled in the art will appreciate that any initiator may be used inlieu, or in addition to, benzyl alcohol. Suitable initiators include,but are not limited to, benzyl alcohol, primary alcohols (e.g., ethanoland butanol), 2-hydroxyethyl acrylate, and 2-hydroxyethyl methacrylate.

Prophetic Example 1 Synthesis of PLA-g-Polystyrene Bottlebrush CopolymerVia ROP Using 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione as aMacromonomer

In this prophetic example, PLA-g-polystyrene bottlebrush copolymer issynthesized using lactide-functionalized polymer3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione as a macromonomer.For this synthesis, as illustrated in FIG. 3, a bromine addition on theL-lactide is employed to synthesize brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione, followed by polymerizationof styrene via ATRP using the brominated lactide monomer as an initiatorto form the lactide-functionalized macromonomer, followed bypolymerization of the lactide-functionalized macromonomer via ROP toform the PLA-g-polystyrene bottlebrush copolymer.

Synthesis of 3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione

To a 1 L three-neck flask are added L-lactide (100.0 g, 0.694 mol),benzene (500 mL) and N-bromosuccimide (NBS) (136.0 g, 0.764 mmol). Themixture is heated to reflux (approximately 80° C.). Mechanical stirringis employed throughout reflux.

A solution of benzoyl peroxide (3.36 g, 13.9 mmol) in benzene (50 mL) isthen added dropwise through a dropping funnel over 20 minutes.

After the monomer is consumed, the reaction mixture is cooled to roomtemperature. Then, filtration is employed to separate the solid filtridefrom the liquid filtrate.

The solid filtride from the filtration is evaporated to dryness forminga pale yellow solid. The solid is dissolved in dichloromethane (750 mL)and the solution is washed with saturated sodium bisulfate solutionthree times and saturated NaCl solution once. The organic layer is driedover MgSO₄, and the solution is evaporated to dryness. The orange solidis recrystallized from ethyl acetate and hexanes to produce 68.9 g ofwhite crystals. One skilled in the art will appreciate thatrecrystallization may be performed in other suitable solutions.

The liquid filtrate from the filtration is evaporated to dryness, andthe solid is recrystallized from ethyl acetate and hexanes to produce27.1 g of white crystals. Here too, one skilled in the art willappreciate that recrystallization may be performed in other suitablesolutions.

The combined yield (from both the solid filtride and the liquidfiltrate) is 96.1. g (62%).

Synthesis of 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione

To a first flask are added CuBr (70.5 mg, 0.49 mmol) andN,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA) (77.5 mg, 0.45 mmol),as well as a magnetic stirrer. The first flask is fitted with a rubberseptum and degassed with three successive cycles of freeze-pump-thaw.Generally, the catalytic complex must be formed with a slight excess ofcopper ([Cu]₀/[PMDETA]₀>1) before exposure to the lactide initiator.Providing a slight excess of copper prevents undesirable side reactions.

To a second flask are added some of the3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione (99.7 mg, 0.45 mmol) preparedin the first step of this example, THF (10 mL), and styrene (5 mL, 43.64mmol). Generally, the ratio of[styrene]₀/[3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione]₀ that may beused ranges from 10 to 200. The second flask is fitted with a rubberseptum and degassed by bubbling with N₂ flow for at least 30 minutes.This mixture is then transferred into the first flask and thepolymerization is carried out under stirring at 70° C. Polymerizationoccurs over a period of 0.5-4 hours. Generally, the polymerization ofstyrene via ATRP may be performed in THF at 60-70° C. for a [styrene]₀of 0.5-5 M and [styrene]₀/[3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione]₀of 10-200.

Copper catalyst is removed by passing the reaction mixture diluted withTHF through an alumina gel column.

The 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione is recovered byprecipitation in 7-fold excess of cold methanol, filtrated and dried upto constant weight.

Synthesis of PLA-g-Polystyrene Bottlebrush Copolymer.

A solution of stannous octoate (Sn(Oct)₂) in anhydrous toluene (0.01 mLof 0.05 M solution) and a solution of benzyl alcohol in anhydroustoluene (0.1 mL of 0.04 M solution) are added to a flask, and thesolvent is removed in vacuo. Some of the lactide-functionalized polymer(macromonomer) (2 g, 0.4 mmol) prepared in the second step of thisexample is added to the flask, along with a magnetic stirrer. The flaskis fitted with a rubber septum and degassed by bubbling with N₂ flow forat least 30 minutes. The polymerization is carried out under stirring at110° C. Polymerization occurs over a period of 5 hours. Generally, thepolymerization of the lactide-functionalized macromonomer via ROP may beperformed in toluene at 110° C. or in the melt at 110-180° C.

The crude PLA-g-polystyrene bottlebrush copolymer is dissolved inchloroform (CHCl₃), recovered by precipitation in cold methanol,filtrated, and dried up to constant weight.

Prophetic Example 2 Synthesis of PLA-g-Polystyrene Graft Copolymer ViaROP Using 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione as aMacromonomer Together with Non-Functionalized Lactide

In this prophetic example, PLA-g-polystyrene graft copolymer issynthesized using lactide-functionalized polymer3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione as a macromonomertogether with non-functionalized lactide. For this synthesis, asillustrated in FIG. 4, a bromine addition on the L-lactide is employedto synthesize brominated lactide monomer3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione, followed by polymerizationof styrene via ATRP using the brominated lactide monomer as an initiatorto form the lactide-functionalized macromonomer, followed bypolymerization of the lactide-functionalized macromonomer andnon-functionalized lactide via ROP to form the PLA-g-polystyrene graftcopolymer.

Synthesis of 3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione.

To a 1 L three-neck flask are added L-lactide (100.0 g, 0.694 mol),benzene (500 mL) and N-bromosuccimide (NBS) (136.0 g, 0.764 mmol). Themixture is heated to reflux (approximately 80° C.). Mechanical stirringis employed throughout reflux.

A solution of benzoyl peroxide (3.36 g, 13.9 mmol) in benzene (50 mL) isthen added dropwise through a dropping funnel over 20 minutes.

After the monomer is consumed, the reaction mixture is cooled to roomtemperature. Then, filtration is employed to separate the solid filtridefrom the liquid filtrate.

The solid filtride from the filtration is evaporated to dryness forminga pale yellow solid. The solid is dissolved in dichloromethane (750 mL)and the solution is washed with saturated sodium bisulfate solutionthree times and saturated NaCl solution once. The organic layer is driedover MgSO₄, and the solution is evaporated to dryness. The orange solidis recrystallized from ethyl acetate and hexanes to produce 68.9 g ofwhite crystals. One skilled in the art will appreciate thatrecrystallization may be performed in other suitable solutions.

The liquid filtrate from the filtration is evaporated to dryness, andthe solid is recrystallized from ethyl acetate and hexanes to produce27.1 g of white crystals. Here too, one skilled in the art willappreciate that recrystallization may be performed in other suitablesolutions.

The combined yield (from both the solid filtride and the liquidfiltrate) is 96.1. g (62%).

Synthesis of 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione.

To a first flask are added CuBr (70.5 mg, 0.49 mmol) andN,N,N′,N′,N-pentamethyldiethylenetriamine (PMDETA) (77.5 mg, 0.45 mmol),as well as a magnetic stirrer. The first flask is fitted with a rubberseptum and degassed with three successive cycles of freeze-pump-thaw.Generally, the catalytic complex must be formed with a slight excess ofcopper ([Cu]₀/[PMDETA]₀>1) before exposure to the lactide initiator.Providing a slight excess of copper prevents undesirable side reactions.

To a second flask are added some of the3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione (99.7 mg, 0.45 mmol) preparedin the first step of this example, THF (10 mL), and styrene (5 mL, 43.64mmol). Generally, the ratio of[styrene]₀/[3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione]₀ that may beused ranges from 10 to 200. The second flask is fitted with a rubberseptum and degassed by bubbling with N₂ flow for at least 30 minutes.This mixture is then transferred into the first flask and thepolymerization is carried out under stirring at 70° C. Polymerizationoccurs over a period of 0.5-4 hours. Generally, the polymerization ofstyrene via ATRP may be performed in THF at 60-70° C. for a [styrene]₀of 0.5-5 M and [styrene]₀/[3-bromo-3,6-dimethyl-1,4-dioxane-2,5-dione]₀of 10-200.

Copper catalyst is removed by passing the reaction mixture diluted withTHF through an alumina gel column.

The 3-poly(styrene)-3,6-dimethyl-1,4-dioxane-2,5-dione is recovered byprecipitation in 7-fold excess of cold methanol, filtrated and dried upto constant weight.

Synthesis of PLA-g-Polystyrene Graft Copolymer.

A solution of stannous octoate (Sn(Oct)₂) in anhydrous toluene (0.01 mLof 0.05 M solution) and a solution of benzyl alcohol in anhydroustoluene (0.1 mL of 0.04 M solution) are added to a flask, and thesolvent is removed in vacuo. Some of the lactide-functionalized polymer(macromonomer) (0.056 g, 0.01 mmol) prepared in the second step of thisexample and non-functionalized lactide (0.056 g, 0.39 mmol) are added tothe flask, along with a magnetic stirrer. The flask is fitted with arubber septum and degassed by bubbling with N₂ flow for at least 30minutes. The polymerization is carried out under stirring at 110° C.Polymerization occurs over a period of 5 hours. Generally, thepolymerization of the lactide-functionalized macromonomer and thenon-functionalized lactide via ROP may be performed in toluene at 110°C. or in the melt at 110-180° C.

The crude PLA-g-polystyrene graft copolymer is dissolved in chloroform(CHCl₃), recovered by precipitation in cold methanol, filtrated, anddried up to constant weight.

One skilled in the art will appreciate that many variations are possiblewithin the scope of the present invention. Thus, while the presentinvention has been particularly shown and described with reference topreferred embodiments thereof, it will be understood by those skilled inthe art that these and other changes in form and details may be madetherein without departing from the spirit and scope of the presentinvention.

What is claimed is:
 1. A polylactic acid-backbone graft copolymerrepresented by the following formula:

wherein R is a hydrogen atom or a methyl group, and wherein R′ is aphenyl group or C(O)OR″, wherein R″ is an alkyl group having one or morecarbon atoms.
 2. The polylactic acid-backbone graft copolymer as recitedin claim 1, wherein the polylactic acid-backbone graft copolymer isrepresented by the following formula:


3. A polylactic acid-backbone bottlebrush copolymer represented by thefollowing formula:

wherein R is a hydrogen atom or a methyl group, and wherein R′ is aphenyl group or C(O)OR″, wherein R″ is an alkyl group having one or morecarbon atoms.
 4. The polylactic acid-backbone bottlebrush copolymer asrecited in claim 3, wherein the polylactic acid-backbone bottlebrushcopolymer is represented by the following formula: